Wall panels or wallboards made from rehydrated stucco conventionally comprise a gypsum core of uniform density sandwiched between two paper cover sheets. Such panels can be mass produced and erected so inexpensively that they have largely replaced prior building techniques using wood panels or plaster. As the usage of such wallboard has expanded, however, specialty used such as walls in high-rise office buildings and apartments have placed a premium on certain properties. Specifically, shaft walls used for example as elevator shafts, air return shafts, and stairwells are subject in some instances to very strict fire regulations. Thus there is a trend in municipal fire codes towards requiring a 0-0-0 fire rating for the exposed surface of elevator shafts. This rating means zero flame spread, zero smoke, and zero toxic gas generation. It has not been possible to achieve such ratings as long as paper-covered wallboard is used, due to the combustibility or at least the smoke-generation capability, of the paper cover sheets. Such paper cover sheets are further troublesome in that they appreciably delay the drying time of the board during its manufacture.
A further problem characteristic of particularly some elevator shafts is that wind loading causes constant flexing of the wallboard. Thus, when used in such walls, the wallboard must have good flexural strength--a feature not present in rehydrated stucco alone due to the low modulus of rupture of the gypsum.
Some presently manufactured wallboard does include various ingredients which give fire resistance to the board. For example, glass fibers on the order of one-half inch long have been incorporated throughout the core of paper-covered gypsum wallboard used to line elevator shaft walls, on a weight percent basis of about 0.25% of the weight of the board. However, such fibers are not long enough to contribute significantly to the flexural strength of the board, as the concentration is insufficient, and at that length, the fibers' tensile strength is greater than their pull-out strength.
Still another approach is illustrated in New Zealand Pat. No. 155,679, which teaches a gypsum panel constructed with glass fibers of various lengths, dispersed generally throughout the rehydrated stucco. Such a construction has eliminated the need for a paper cover sheet. However, the process of making such panels is difficult, time-consuming, and involves the use of a considerable amount of glass fibers, inasmuch as they are distributed more or less uniformly throughout the board or panel.